1. Field of the Invention
The invention relates to a rotary head for scanning the surface of elongated metal test materials and, more particularly to such a rotary head in a non-destructive eddy current defect test apparatus.
2. Description of Related Art
For many years, rotary heads have been known for use in non-destructive testing of materials, and are widely used. They serve there for testing, in particular, of bare curved surface materials of steel and non-ferrous substances to detect defects located at any point below the surface and extending up to the surface. During long periods of application, such rotary heads have gained a high technological performance providing excellent results. On bare surfaces, e.g., cracks having a depth of only 30 micrometers can be reliably detected, and this accomplished at rather high running speeds.
On the other hand, there are present limits, which would be desirable to overcome, since this would lead to further possible applications. For instance, sensitive testing of "non-bare" ferromagnetic steel is not possible with known eddy current rotary heads because of the high interference level encountered. Another example relates to cracks in ferromagnetic materials, wherein further processing, as rolling, drawing or peeling, will result in a surface being closed again. In this case, the low penetration depth of eddy currents for the high frequencies required to achieve desired sensitivity accounts for the fact that such "smeared" cracks are impossible to detect in most cases. Difficulties have also been encountered in the testing of welded austenitic pipes, wherein ferrous impurities are found to generate a high interference level in the area of the weld seam, and, therefore, prevent an efficient testing of such pipes.